Iron tungsten coating formulations and processes

ABSTRACT

An electrolyte solution for iron-tungsten plating is prepared by dissolving in an aqueous medium a divalent iron salt (e.g., iron (II) sulfate) and an alkali metal citrate (e.g., sodium citrate, potassium citrate, or other alkali metal citrate) to form a first solution, dissolving in the first solution a tungstate salt (e.g., sodium tungstate, potassium tungstate, or other potassium tungstate) to form a second solution, and dissolving in the second solution a citric acid to form the electrolyte solution. An iron-tungsten coating is formed on a substrate using the electrolyte solution by passing a current between a cathode and an anode through the electrolyte solution to deposit iron and tungsten on the substrate.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation, and claims the benefit, of U.S.patent application Ser. No. 15/618,850, presently pending and filed onJun. 9, 2017, which is hereby incorporated by reference in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to coatings and, more particularly, toproviding iron-tungsten coatings on substrates.

2. Related Art

Chrome plating is an electroplating process that provides a chromecoating on a substrate. Hard chrome plating provides a chrome coatinghaving a thickness typically about 10 microns or greater, therebyproviding hardness and wear resistance to the coated substrate. Hardchrome coatings are plated from baths containing chromic acid andcatalysts based on fluorides, sulfates, or organic acids. However, suchbaths contain chromium in its hexavalent form (Cr-VI).

Baths with chromium in its trivalent form (Cr-III) are used fordecorative chrome plating, which provides a chrome coating having athickness typically ranging from about 0.1 to about 0.5 microns.However, there are challenges to providing thicker, hard, and functionalchrome coatings using trivalent chromium baths.

Nickel-tungsten alloy coatings have been considered as an alternative tohard chrome coatings. However, nickel-based baths used fornickel-tungsten alloy coatings involve the use of nickel and often useboric acid as a buffering agent.

Thus, there is a need for improved coating methods and formulations ofsolutions used for coating of substrates.

SUMMARY

In accordance with embodiments of the present disclosure, variousmethods and formulations are provided for plating a substrate to providean iron-tungsten coating. Advantageously, the electrolyte solution usedfor the plating does not include any chromium, boric acid, or nickel,while still resulting in a coating that is structurally robust andreliable, yet cost-effective. Thus, the methods and formulationsdescribed herein can advantageously be used for plating to form bright,hard coatings (e.g., robust and functional iron-tungsten alloycoatings). However, the present disclosure is not limited to bright,hard coatings and the methods and formulations described herein can alsobe advantageously used to effectively and efficiently provide dullcoatings (e.g., dark or black coatings), which can be used, for example,when a surface that absorbs light is desirable (e.g., solar panels). Theblack coating, though not as hard as the bright coating, issignificantly harder than the substrates such as mild steel or copper.

In one example embodiment, a method for plating a substrate using anelectrolyte solution includes dissolving in an aqueous medium a divalentiron salt in an amount ranging from about 0.05 to about 0.5 mol perliter of the electrolyte solution and an alkali metal citrate in anamount ranging from about 0.05 to about 2 mol per liter of theelectrolyte solution to form a first solution, dissolving in the firstsolution a tungstate salt in an amount ranging from about 0.1 to about1.5 mol per liter of the electrolyte solution to form a second solution,dissolving in the second solution a citric acid in an amount rangingfrom about 0.01 to about 1 mol per liter of the electrolyte solution toform the electrolyte solution, and passing a current between a cathodeand an anode through the electrolyte solution to deposit iron andtungsten on the substrate.

In certain aspects, the step of dissolving the divalent iron salt andthe alkali metal citrate includes dissolving iron (II) sulfate in anamount ranging from about 0.05 to about 0.2 mol per liter of theelectrolyte solution and sodium citrate or potassium citrate in anamount ranging from about 0.15 to about 0.5 mol per liter of theelectrolyte solution. The step of dissolving the tungstate salt includesdissolving sodium tungstate or potassium tungstate in an amount rangingfrom about 0.15 to about 0.5 mol per liter of the electrolyte solution.The step of dissolving the citric acid includes dissolving the citricacid in an amount ranging from about 0.02 to about 0.5 mol per liter ofthe electrolyte solution.

In certain aspects, the step of passing the current is performed using acarbonaceous anode, a graphite anode, a platinum anode, or a platinizedtitanium anode to deposit iron and tungsten. The substrate on which ironand tungsten is deposited includes a steel substrate, a coppersubstrate, a brass substrate, a copper-coated substrate, a nickel-coatedsubstrate, or a combination thereof.

In certain aspects, the method further includes dissolving an ammoniumhalide (e.g., ammonium chloride) in an amount ranging from about 0.1 toabout 0.4 mol per liter of the electrolyte solution and an alkali metalhalide (e.g., sodium bromide) in an amount ranging from 0.03 to about0.15 mol per liter of the electrolyte solution. With the addition of theammonium halide and alkali metal halide, the step of passing the currentforms a dull-bright coating layer.

In certain aspects, the step of passing the current includes applying adirect current having a current density ranging from about 0.002 toabout 0.04 A/cm² to form a bright hard coating layer including an alloyof iron and tungsten. Alternatively, the step of passing the currentincludes applying a direct current having a current density ranging fromabout 0.05 to about 0.1 A/cm² to form a dull coating layer including analloy of iron and tungsten.

In certain aspects, the step of dissolving the tungstate salt includesdissolving the tungstate salt in an amount ranging from about 0.3 toabout 0.45 mol per liter of the electrolyte solution such that the stepof passing the current forms a bright hard coating layer including analloy of iron and tungsten. Alternatively, the step of dissolving thetungstate salt includes dissolving the tungstate salt in an amountranging from about 0.1 to about 0.3 mol per liter of the electrolytesolution such that the step of passing the current forms a dull coatinglayer including an alloy of iron and tungsten.

In certain aspects, the method further includes maintaining atemperature of about 50 to about 70° C. during the step of passing thecurrent such that the step of passing the current forms a bright hardcoating layer including an alloy of iron and tungsten. Alternatively,the method further includes maintaining a temperature of about 20 toabout 50° C. during the step of passing the current such that the stepof passing the current forms a dull coating layer including an alloy ofiron and tungsten.

In certain aspects, the method further includes maintaining a pH rangingfrom about 7 to about 12 during the step of passing the current suchthat the step of passing the current forms a bright hard coating layerincluding an alloy of iron and tungsten. Alternatively, the methodfurther includes maintaining a pH ranging from about 3 to about 7 duringthe step of passing the current, wherein the step of passing the currentforms a dull coating layer including an alloy of iron and tungsten.

In certain aspects, the step of dissolving the citric acid includesdissolving the citric acid in an amount ranging from about 0.05 to about0.25 mol per liter of the electrolyte solution such that the step ofpassing the current forms a bright hard coating layer including an alloyof iron and tungsten. Alternatively, the step of dissolving the citricacid includes dissolving the citric acid in an amount ranging from about0.25 to about 0.5 mol per liter of the electrolyte solution such thatthe step of passing the current forms a dull coating layer including analloy of iron and tungsten.

In certain aspects, the step of passing the current forms a plurality ofiron-tungsten alloy coating layers including at least one bright hardcoating layer and at least one dull coating layer. In certain additionalaspects, the step of applying the plurality of currents includesapplying a first direct or pulsed current having a first current densityduring at least one first time interval to form the at least one brighthard coating layer, and applying a second direct or pulsed currenthaving a second current density during at least one second time intervalto form the at least one dull coating layer.

In another example embodiment, a product including one or moreiron-tungsten alloy coating layers is formed by any of the methodsabove. In certain aspects, the product includes at least one bright hardcoating layer including an alloy of iron and tungsten having a tungstencontent ranging from about 18 to about 28 At % (atomic %). In addition,or alternatively, the product includes at least one dull coating layerincluding an alloy of iron and tungsten having a tungsten contentranging from about 5 to about 18 At % or about 28 to about 40 At %.

In yet another example embodiment, an electrolyte solution includes adivalent iron salt in an amount ranging from about 0.05 to about 0.5 molper liter of the electrolyte solution, an alkali metal citrate in anamount ranging from about 0.05 to about 2 mol per liter of theelectrolyte solution, an alkali tungstate metal in an amount rangingfrom about 0.1 to about 1.5 mol per liter of the electrolyte solution,and citric acid in an amount ranging from about 0.01 to about 1 mol perliter of the electrolyte solution.

In certain aspects, the divalent iron salt includes iron (II) sulfate inan amount ranging from about 0.05 to about 0.2 mol per liter of theelectrolyte solution. The alkali metal citrate includes sodium citrateor potassium citrate in an amount ranging from about 0.15 to about 0.5mol per liter of the electrolyte solution. The tungstate salt includessodium tungstate or potassium tungstate in an amount ranging from about0.15 to about 0.5 mol per liter of the electrolyte solution. The citricacid is included in an amount ranging from about 0.02 to about 0.5 molper liter of the electrolyte solution. The electrolyte solution furtherincludes an ammonium halide (e.g., ammonium chloride) in an amountranging from about 0.1 to about 0.4 mol per liter of the electrolytesolution and an alkali metal halide (e.g., sodium bromide) in an amountranging from 0.03 to about 0.15 mol per liter of the electrolytesolution.

The scope of the invention is defined by the claims, which areincorporated into this section by reference. A better understanding ofthe methods and formulations for iron-tungsten coating of the presentdisclosure, as well as an appreciation of the above and additionaladvantages thereof, will be afforded to those of skill in the art by aconsideration of the following detailed description of one or moreexample embodiments thereof. In this description, reference is made tothe various views of the appended sheets of drawings, which are brieflydescribed below, and within which, like reference numerals are used toidentify like ones of the elements illustrated therein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example process for forming an iron-tungstencoating on a substrate in accordance with an embodiment of the presentdisclosure.

FIG. 2A is a diagrammatic cross-section view of a bright hard coatingformed on a substrate in accordance with an embodiment of the presentdisclosure.

FIG. 2B is a diagrammatic cross-section view of a dull coating formed ona substrate in accordance with an embodiment of the present disclosure.

FIG. 3 is a graph showing the characteristics of iron-tungsten coatingsformed using various current densities in accordance with an embodimentof the present disclosure.

FIG. 4 is a graph showing the characteristics of iron-tungsten coatingsformed with an electrolyte solution containing various amounts oftungstate salt in accordance with an embodiment of the presentdisclosure.

FIG. 5 is a graph showing the characteristics of iron-tungsten coatingsformed at various temperatures in accordance with an embodiment of thepresent disclosure.

FIG. 6 is a graph showing the characteristics of iron-tungsten coatingsformed by maintaining the electrolyte solution at various pH values inaccordance with an embodiment of the present disclosure.

FIG. 7 is a graph showing the characteristics of iron-tungsten coatingsformed with an electrolyte solution containing various amounts of acitric acid in accordance with an embodiment of the present disclosure.

FIG. 8 is a graph showing the hardness of iron-tungsten coatings withvarious tungsten content in accordance with an embodiment of the presentdisclosure.

FIG. 9 is a graph showing the modulus of iron-tungsten coatings withvarious tungsten content in accordance with an embodiment of the presentdisclosure.

FIG. 10A is a diagrammatic cross-section view of a bright hard coatinglayer formed on a substrate and a dull coating layer formed on thebright hard coating layer in accordance with an embodiment of thepresent disclosure.

FIG. 10B is a diagrammatic cross-section view of a dull coating layerformed on a substrate and a bright hard coating layer formed on the dullcoating layer in accordance with an embodiment of the presentdisclosure.

FIG. 10C is a diagrammatic cross-section view of a plurality of coatinglayers formed on a substrate that includes at least one bright hardcoating layer and at least one dull coating layer in accordance with anembodiment of the present disclosure.

FIG. 11 illustrates an example process for forming an iron-tungstencoating on a substrate using an electrolyte solution with additives inaccordance with an embodiment of the present disclosure.

FIG. 12A is an image of a bright hard iron-tungsten coating formed on amild steel alloy in accordance with an embodiment of the presentdisclosure.

FIG. 12B is an image of a dull iron-tungsten coating formed on a mildsteel alloy in accordance with an embodiment of the present disclosure.

FIG. 13A is a scanning electron microscopy (SEM) image of the brighthard coating of FIG. 12A formed using a current density of about 0.01A/cm².

FIG. 13B is an SEM image of an iron-tungsten coating formed using acurrent density of about 0.025 A/cm² in accordance with an embodiment ofthe present disclosure.

FIG. 13C is an SEM image of an iron-tungsten coating formed using acurrent density of about 0.05 A/cm² in accordance with an embodiment ofthe present disclosure.

FIG. 13D is an SEM image of the black coating of FIG. 12B formed using acurrent of about 0.075 A/cm².

FIG. 14 is an SEM image of a cross-section of the bright hard coating ofFIG. 12A in accordance with an embodiment of the present disclosure.

FIG. 15A is an image of a bright hard iron tungsten coating formed on abrass substrate in accordance with an embodiment of the presentdisclosure.

FIG. 15B is an image of a dull iron-tungsten coating formed on a brasssubstrate in accordance with an embodiment of the present disclosure.

FIG. 16A is an SEM image of the bright hard coating of FIG. 15A formedusing a current density of about 0.01 A/cm².

FIG. 16B is an SEM image of the dull coating of FIG. 15B formed using acurrent of about 0.075 A/cm².

FIG. 17 is an image of a bright hard iron-tungsten coating formed on asubstrate using pulsed current in accordance with an embodiment of thepresent disclosure.

FIG. 18 is an SEM image of the bright hard coating of FIG. 17.

FIG. 19 is an energy-dispersive X-ray spectroscopy spectrum of thebright hard coating of FIG. 17.

FIG. 20A is an image of a 1 ampere Hull cell panel coated using theelectrolyte solution prepared as shown in FIG. 1.

FIG. 20B is an image of a 1 ampere Hull cell panel coated using theelectrolyte solution prepared as shown in FIG. 11.

FIG. 20C is an image of a 5 ampere Hull cell panel coated using theelectrolyte solution prepared as shown in FIG. 11.

DETAILED DESCRIPTION

FIG. 1 illustrates an example process 100 for forming an iron-tungstencoating (e.g., an iron-tungsten alloy coating or other iron-tungstencontaining coating) on a substrate (e.g., a steel substrate, a coppersubstrate, a brass substrate, a copper-coated substrate, a nickel-coatedsubstrate, or other metal or metal alloy substrate). Process 100includes preparing an electrolyte solution and passing current betweenan anode and a cathode in the electrolyte solution. The compound of thefirst block is dissolved in an aqueous medium such as water, and one ormore respective compounds of each subsequent block is dissolved in thesolution resulting from the previously performed block to form theelectrolyte solution.

At block 102, a divalent iron salt is dissolved. The divalent iron saltis a divalent iron source. The divalent iron salt includes iron (II)sulfate, iron (II) halide (e.g., iron (II) bromide, iron (II) chloride,iron (II) iodide, or other iron (II) halide), iron (II) nitrate, iron(II) acetate, iron (II) perchlorate, and/or other divalent iron salt. Incertain aspects, each of these divalent iron salts includes itsrespective hydrated forms. For example, iron (II) sulfate has theformula FeSO₄.xH₂O, where x is a whole number (e.g., 0, 1, 2, 4, 5, 6,or 7). Accordingly, iron (II) sulfate is, for example, anhydrous iron(II) sulfate, iron (II) sulfate monohydrate, iron (II) sulfatedihydrate, iron (II) sulfate tetrahydrate, iron (II) sulfatepentahydrate, iron (II) sulfate hexahydrate, iron (II) sulfateheptahydrate, or iron (II) sulfate with another hydration state. Incertain aspects, the amount of the divalent iron salt that is dissolvedranges from about 0.05 mol (moles) to about 0.5 mol per liter of theelectrolyte solution to be formed. The amount of the divalent iron saltthat is dissolved is, for example, about 0.05 mol, 0.1 mol, 0.15 mol,0.2 mol, 0.25 mol, 0.3 mol, 0.35 mol, 0.4 mol, 0.45 mol, or 0.5 mol perliter of the electrolyte solution, where any value can form an upper endpoint or a lower end point, as appropriate. The term “about,” as usedherein when referring to a measurable value such as an amount,concentration, time, and the like, is meant to encompass variations of±20%, ±10%, ±5%, ±1%, ±0.5%, or even ±0.1% of the specified value.

The divalent iron salt is dissolved, for example, by stirring at ambienttemperature, at room temperature, at about 25° C., or at a temperatureranging from about 20° C. to about 30° C. The stirring is performed, forexample, for about 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25minutes, 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55minutes, or 60 minutes, where any value can form an upper end point or alower end point, as appropriate, or until all the divalent iron salt hasbeen dissolved. The temperature at which block 102 is performed is, forexample, about 10° C., 15° C., 20° C., 25° C., 30° C., 35° C., or 40°C., where any value can form an upper end point or a lower end point, asappropriate.

At block 104, an alkali metal citrate is dissolved. The alkali metalcitrate includes sodium citrate (e.g., trisodium citrate, disodiumhydrogen citrate, sodium dihydrogen citrate, or other sodium citrate),potassium citrate, and/or other alkali metal citrate. In certainaspects, each of these alkali metal citrates includes its respectivehydrated forms. For example, trisodium citrate has the formulaNa₃C₆H₅O₇.xH₂O, where x is a whole number (e.g., 0 or 2). Accordingly,trisodium citrate is, for example, anhydrous trisodium citrate,trisodium citrate dihydrate, or trisodium citrate with another hydrationstate.

In certain aspects, the amount of the alkali metal citrate that isdissolved ranges from about 0.05 mol to about 2 mol per liter of theelectrolyte solution to be formed. The amount of the alkali metalcitrate that is dissolved is, for example, about 0.05 mol, 0.1 mol, 0.2mol, 0.3 mol, 0.4 mol, 0.5 mol, 0.6 mol, 0.7 mol, 0.8 mol, 0.9 mol, 1.0mol, 1.1 mol, 1.2 mol, 1.3 mol, 1.4 mol, 1.5 mol, 1.6 mol, 1.7 mol, 1.8mol, 1.9 mol, or 2.0 mol per liter of the electrolyte solution, whereany value can form an upper end point or a lower end point, asappropriate.

The alkali metal citrate is dissolved, for example, by stirring atambient temperature, at room temperature, at about 25° C., or at atemperature ranging from about 20° C. to about 30° C. The stirring isperformed, for example, for about 5 minutes, 10 minutes, 15 minutes, 20minutes, 25 minutes, 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50minutes, 55 minutes, or 60 minutes, where any value can form an upperend point or a lower end point, as appropriate, or until all the alkalimetal citrate has been dissolved. The temperature at which block 104 isperformed is, for example, about 10° C., 15° C., 20° C., 25° C., 30° C.,35° C., or 40° C., where any value can form an upper end point or alower end point, as appropriate.

At block 106, a tungstate salt is dissolved. The tungstate salt includesan alkali metal tungstate (e.g., sodium tungstate, potassium tungstate,or other alkali metal tungstate), and/or other tungstate salt. Incertain aspects, each of these tungstate salts includes its respectivehydrated forms. For example, sodium tungstate has the formulaNa₂WO₄.xH₂O, where x is a whole number (e.g., 0 or 2). Accordingly,sodium tungstate is, for example, anhydrous sodium tungstate, sodiumtungstate dihydrate, or sodium tungstate with another hydration state.

The tungstate salt is dissolved, for example, by stirring at ambienttemperature, at room temperature, at about 25° C., or at a temperatureranging from about 20° C. to about 30° C. The stirring is performed, forexample, for about 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25minutes, 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55minutes, or 60 minutes, where any value can form an upper end point or alower end point, as appropriate, or until all the tungstate salt hasbeen dissolved. The temperature at which block 104 is performed is, forexample, about 10° C., 15° C., 20° C., 25° C., 30° C., 35° C., or 40°C., where any value can form an upper end point or a lower end point, asappropriate.

At block 108, a citric acid is dissolved. In certain aspects, the citricacid includes its hydrated forms. The citric acid has the formulaC₆H₈O₇.xH₂O, where x is a whole number (e.g., 0 or 1). The citric acidis, for example, anhydrous citric acid, citric acid monohydrate, or acitric acid with another hydration state.

In certain aspects, the amount of the citric acid that is dissolvedranges from about 0.01 mol to about 1.0 mol of the electrolyte solutionto be formed. The amount of the alkali metal sulfate that is dissolvedis, for example, about 0.1 mol, 0.2 mol, 0.3 mol, 0.4 mol, 0.5 mol, 0.6mol, 0.7 mol, 0.8 mol, 0.9 mol, 1.0 mol, 1.1 mol, 1.2 mol, 1.3 mol, 1.4mol, 1.5 mol, 1.6 mol, 1.7 mol, 1.8 mol, 1.9 mol, or 2.0 mol per literof the electrolyte solution, where any value can form an upper end pointor a lower end point, as appropriate.

The citric acid is dissolved, for example, by stirring for 15 minutes atambient temperature, at room temperature, at about 25° C., or at atemperature ranging from about 20° C. to about 30° C. The stirring isperformed, for example, for about 5 minutes, 10 minutes, 15 minutes, 20minutes, 25 minutes, or 30 minutes, where any value can form an upperend point or a lower end point, as appropriate, or until all the citricacid has been dissolved. The temperature at which block 108 is performedis, for example, about 10° C., 15° C., 20° C., 25° C., 30° C., 35° C.,or 40° C., where any value can form an upper end point or a lower endpoint, as appropriate.

At block 110, the pH is adjusted and/or maintained. The pH isadjusted/maintained using one or more acids or bases, such as potassiumhydroxide (KOH), sodium hydroxide (NaOH), and/or sulfuric acid (H₂SO₄).In certain aspects, the pH of the electrolyte solution is adjustedto/maintained at a target pH within the range from about 3 to about 12(e.g., about 7.5 or a pH ranging from about 7 to 8). The pH is adjustedto/maintained, for example, at about 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7,7.5, 8, 8.5, 9, 9.5, 10, 10.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, or13, where any value can form an upper end point or a lower end point, asappropriate. In an example, the pH is adjusted before passing thecurrent at block 112. In another example, the pH is maintained at atarget pH or a target pH range during the passing of the current atblock 112. In a further example, the pH is adjusted before andmaintained during the passing of the current at block 112.

In addition, or in the alternative, a temperature of the electrolytesolution is adjusted and/or maintained. In certain aspects, thetemperature is adjusted to/maintained at a target temperature within therange from about 20° C. to about 70° C. The target temperature is, forexample, about 20° C., 25° C., 30° C., 35° C., 40° C., 45° C., 50° C.,55° C., 60° C., 65° C., or 70° C., where any value can form an upper endpoint or a lower end point, as appropriate. In an example, thetemperature is adjusted before passing the current at block 112. Inanother example, the temperature is maintained during the passing of thecurrent at block 112. In a further example, the temperature is adjustedbefore and maintained during passing the current at block 112.

At block 112, a cathode and an anode are placed in the electrolytesolution, the cathode including the substrate, and a current is passedbetween the cathode and the anode through the electrolyte solution todeposit iron and tungsten on the substrate. In certain aspects, thesubstrate is a steel substrate, a copper substrate, a brass substrate, anickel substrate, a copper-coated substrate, or a nickel-coatedsubstrate. However, other substrates are contemplated as one skilled inthe art will appreciate.

In certain aspects, the anode includes a carbonaceous electrodematerial. For example, the carbonaceous anode is a graphite anode orother anode that includes carbon. Advantageously, the graphite anode orother carbonaceous anode minimizes gas evolution and formation ofundesirable byproducts, as well as facilitating a desirable depositionrate (e.g., ranging from about 1 microns to about 2 microns per minute).Alternatively, a platinum anode or a platinized titanium anode is used.

In some embodiments, direct current is used. In certain aspects, thedirect current provides a current density ranging from about 0.002 A/cm²to about 0.1 A/cm². The value of the current density can be adjusteddepending on the separation between the cathode and anode and thedesired type or characteristics of the coating to be formed. The currentdensity is, for example, about 0.002 A/cm², 0.004 A/cm², 0.006 A/cm²,0.008 A/cm², 0.01 A/cm², 0.02 A/cm², 0.03 A/cm², 0.04 A/cm², 0.05 A/cm²,0.06 A/cm², 0.07 A/cm², 0.08 A/cm², 0.08 A/cm², 0.09 A/cm², or 0.1A/cm², where any value can form an upper end point or a lower end point,as appropriate, depending on the separation between the cathode andanode.

In other embodiments, pulsed current is used. In certain aspects, thepulsed current provides an average current density ranging from aboutranging from about 0.002 A/cm² to about 0.1 A/cm². The value of theaverage current density can be adjusted depending on the separationbetween the cathode and anode. The peak current density can be twice ofthe average current density. The average current density is, forexample, about 0.002 A/cm², 0.004 A/cm², 0.006 A/cm², 0.008 A/cm², 0.01A/cm², 0.02 A/cm², 0.03 A/cm², 0.04 A/cm², 0.05 A/cm², 0.06 A/cm², 0.07A/cm², 0.08 A/cm², 0.08 A/cm², 0.09 A/cm², or 0.1 A/cm², where any valuecan form an upper end point or a lower end point, as appropriate,depending on the separation between the cathode and anode.

In certain aspects, the pulsed current has a duty cycle ranging fromabout 20% to about 90%. The duty cycle is, for example, about 20%, 25%,30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, or 90%,where any value can form an upper end point or a lower end point, asappropriate. In certain aspects, the pulsed current has a frequencyranging from about 10 Hz to about 100 Hz. The frequency is, for example,about 10 Hz, 20 Hz, 30 Hz, 40 Hz, 50 Hz, 60 Hz, 70 Hz, 80 Hz, 90 Hz, or100 Hz, where any value can form an upper end point or a lower endpoint, as appropriate. For example, if the pulsed current has a dutycycle of about 40% and a frequency of about 25 Hz, the ON time is about16 milliseconds and the OFF time is about 24 milliseconds.

In response to performing block 112, iron and tungsten is deposited onthe substrate. Block 112 can be performed until an iron-tungsten coatinglayer having a desired thickness (e.g., a thickness greater than about 5microns) is formed on the substrate. The iron-tungsten coating layer canbe a bright hard coating as shown in FIG. 2A or a dull coating as shownin FIG. 2B depending on various conditions as shown in FIGS. 3-7 anddescribed in further detail below.

In some embodiments, process 100 is performed in the order presented. Inother embodiments, process 100 is performed in a different order. Someblocks can be performed in order while other blocks are performed in adifferent order. For example, blocks 102 and 104 can be performed inorder, in reverse order, or concurrently, while blocks 106 and 108 areperformed in order after blocks 102 and 104. In another example, block110 can be performed before 112 or block 110 can be performed throughoutblock 112. Other orders are contemplated as one skilled in the art willappreciate. Further, one or more of the blocks (e.g., block 110) can beomitted in some embodiments.

FIG. 2A shows a substrate 202 with a bright hard coating layer 204.Bright hard coating layer 204 is formed, for example, by process 100 ofFIG. 1, in which at block 112 a direct current with a current density(or a pulsed current with an average current density) ranging from about0.002 A/cm² to about 0.04 A/cm² is applied when the cathode and theanode is separated by about 3.5 cm. Bright hard coating layer 204 islustrous and has a hardness greater than, for example, about 700 HV.

FIG. 2B shows a substrate 212 with a dull coating layer 214 (e.g., adark coating layer or a black coating layer). Dull coating layer 214 isformed, for example, by process 100 of FIG. 1, in which at block 112 adirect current with a current density (or a pulsed current with anaverage current density) ranging from about 0.05 A/cm² to about 0.1A/cm² is applied when the cathode and the anode are separated by about3.5 cm to form dull coating layer 214. Dull coating layer 214 lacksluster compared to bright hard coating layer 204.

FIGS. 3-7 are graphs showing characteristics of iron-tungsten coatingsas functions of conditions relating to the electrolyte solution orparameters for the current during plating. The conditions includebrightness (bright vs. dull), hardness, tungsten content W, and currentefficiency.

FIG. 3 is a graph showing the characteristics of iron-tungsten coatingsformed using various current densities. An iron-tungsten coating layerdeposited using a low current density ranging from about 0.002 A/cm² toabout 0.04 A/cm² (e.g., about 0.01 A/cm² or about 0.025 A/cm²) at block112 of FIG. 1 is hard and bright. A thin bright hard coating can be usedfor decorative purposes in place of decorative chrome plating, while athick bright hard coating can be used as an alternative to hard chromeplating. Further, an iron-tungsten coating layer deposited using ahigher current density ranging from about 0.05 A/cm² to about 0.1 A/cm²(e.g., about 0.05 A/cm² or about 0.075 A/cm²) at block 112 of FIG. 1 isdull (e.g., dark or black). A dull coating can be used as solarabsorptive coatings as an alternative to black chrome plating. A dullcoating can also be used for decorative purposes. Although directcurrent was used for the current densities of FIG. 3, pulsed current canalso be used in other embodiments.

FIG. 4 is a graph showing the characteristics of iron-tungsten coatingsformed with an electrolyte solution containing various amounts oftungstate salt. An iron-tungsten coating layer formed using anelectrolyte solution in which the tungstate salt in an amount rangingfrom about 0.3 to about 0.45 mol per liter of the electrolyte solution(e.g., about 110 grams of sodium tungstate per liter of the electrolytesolution or about 130 g of sodium tungstate per liter of the electrolytesolution) is dissolved at block 106 of FIG. 1 results in a bright hardcoating layer. Further, iron-tungsten coating layers formed using anelectrolyte solution in which the tungstate salt in an amount rangingfrom about 0.1 to about 0.3 mol per liter of the electrolyte solution(e.g., about 50 grams of sodium tungstate per liter of the electrolytesolution or about 80 g of sodium tungstate per liter of the electrolytesolution) is dissolved at block 106 of FIG. 1 results in a dull coatinglayer.

FIG. 5 is a graph showing the characteristics of iron-tungsten coatingsformed at various temperatures. An iron-tungsten coating layer depositedwhile maintaining the temperature of the electrolyte solution at atemperature ranging from about 50° C. to about 70° C. (e.g., about 60°C. or about 70° C.) at block 110 of FIG. 1 results in a bright hardcoating layer. Further, an iron-tungsten coating layer deposited whilemaintaining the temperature of the electrolyte solution at a temperatureranging from about 20° C. to about 50° C. (e.g., about 25° C. or about40° C.) at block 110 of FIG. 1 results in a dull coating layer.

FIG. 6 is a graph showing the characteristics of iron-tungsten coatingsformed by maintaining the electrolyte solution at various pH values. Aniron-tungsten coating layer deposited while maintaining the pH of theelectrolyte solution at a pH ranging from about 7 to about 12 (e.g.,about 9 or about 12) at block 110 of FIG. 1 results in a bright hardcoating layer. Further, an iron-tungsten coating layer deposited whilemaintaining the pH of the electrolyte solution at a pH ranging fromabout 3 to about 7 (e.g., about 3 or about 5) at block 110 of FIG. 1results in a dull coating layer.

FIG. 7 is a graph showing the characteristics of iron-tungsten coatingsformed with an electrolyte solution containing various amounts of acitric acid. An iron-tungsten coating layer formed using an electrolytesolution in which the citric acid in an amount ranging from about 0.25mol to about 0.5 mol per liter of the electrolyte solution (e.g., about30 grams of anhydrous citric acid per liter of the electrolyte solution)is dissolved at block 108 of FIG. 1 results in a bright hard coatinglayer. Further, iron-tungsten coating layers formed using an electrolytesolution in which the citric acid in an amount ranging from about 0.1 toabout 0.3 mol per liter of the electrolyte solution (e.g., about 50grams of anhydrous citric acid per liter of the electrolyte solution orabout 90 g of anhydrous citric acid per liter of the electrolytesolution) is dissolved at block 108 of FIG. 1 results in a dull coatinglayer.

FIG. 8 is a graph showing the hardness of iron-tungsten coatings withvarious tungsten content, and FIG. 9 is a graph showing the modulus ofiron-tungsten coatings with various tungsten content. The hardness andthe modulus of the iron-tungsten coating depends on the ratio betweeniron and tungsten (“the Fe:W ratio”). The Fe:W ratio can be tailored bychanging electroplating parameters such as current density, pH, ortemperature (as shown in FIGS. 3, 5, and 6). Thus, iron-tungstencoatings with different Fe/W compositions and properties can be formedfrom the same electrolyte solution. Alternatively, or in addition, theFe:W ratio can be tailored by changing the electrolyte solution contentas shown in FIGS. 4 and 7).

The iron-tungsten coatings is a bright hard coating or a dull coatingbased on the Fe:W ratio. Accordingly, in some aspects the bright hardcoating is defined by its tungsten content, for example, aniron-tungsten coating having a tungsten content ranging from about 18 At% to about 28 At %. The dull coating is defined by its tungsten content,for example, an iron-tungsten coating having a tungsten content rangingfrom about 5 At % to about 18 At % or about 28 At % to about 40 At %.This can be in addition to or alternatively from defining the brighthard coating and the dull coating in terms of its luster or lackthereof.

FIGS. 10A-10C are diagrammatic cross-sectional views of a plurality ofcoating layers formed on substrates. A plurality of coating layers isformed on a substrate, for example, by varying the current at block 112of FIG. 1.

FIG. 10A is a diagrammatic cross-section view of a substrate 1002 with abright hard coating layer 1004 formed on substrate 1002 and a dullcoating layer 1006 formed on bright hard coating layer 1004. Forexample, at block 112 of FIG. 1, a current density of about 0.01 A/cm²is applied for a first time period to form bright hard coating layer1004 having a tungsten content of about 23 At %, and then a currentdensity of about 0.07 A/cm² is applied for a second time period to formdull coating layer 1006 having a tungsten content of about 30 At %.

FIG. 10B is a diagrammatic cross-section view of a substrate 1012 with adull coating layer 1014 formed on substrate 1012 and a bright hardcoating layer 1016 formed on bright hard coating layer 1014. Forexample, at block 112 of FIG. 1, a current density of about 0.05 A/cm²is applied for a first time period to form dull coating layer 1014having a tungsten content of about 30 At %, and then a current densityof about 0.01 A/cm² is applied for a second time period to form brighthard coating layer 1006 having a tungsten content of about 23 At %.

FIG. 10C is a diagrammatic cross-section view of a substrate 1022 with aplurality of coating layers 1024, 1026, 1028, and 1030 formed onsubstrate 1022. For example, at block 112 of FIG. 1, a current densityof about 0.01 A/cm² is applied for a first time period to form brighthard coating layer 1024 having a tungsten content of about 23 At %, thena current density of about 0.025 A/cm² is applied for a second timeperiod to form bright hard coating layer 1026 having a tungsten contentof about 25 At %, then a current density of about 0.04 A/cm² is appliedfor a third time period to get a dark dull layer having a tungstencontent of about 28 At %, and a then a current density of about 0.07A/cm² is applied for a fourth time period to get a black dull layerhaving a tungsten content of about 30 At %. Although four steps ofcurrent densities/time periods are used for the coating in FIG. 10C, anynumber of current densities/time periods can be used to provide thedesired number of coating layers in other examples.

The coating layers of FIGS. 10A-10C can be formed by varying otherelectroplating parameters or by changing the electrolyte solution duringplating. Further, in certain aspects an electroplating parameter such ascurrent density is varied in steps (as described in relation to FIGS.10A-10C above). Alternatively, the electroplating parameter is variedgradually. The current density can be increased each step (e.g., asdescribed in relation to FIG. 10C above), the current density can bedecreased each step, or the current density can vary without such trendsin other examples. The layers are equal in thickness as shown in FIG.10A or, alternatively, the layers have different thicknesses as shown inFIGS. 10B-10C.

FIG. 11 illustrates an example process 1100 for forming an iron-tungstencoating on a substrate using an electrolyte solution with additives.Blocks 1102, 1104, 1106, 1108, 1112, and 1114 are as described in blocks102, 104, 106, 108, 110, and 112, respectively. At block 1110, additivessuch as an ammonium halide (e.g., ammonium chloride) and/or an alkalimetal halide (e.g., sodium bromide) are dissolved. In certain aspects,an ammonium halide in an amount ranging from about 0.1 to about 0.4 molper liter of the electrolyte solution and an alkali metal halide in anamount ranging from 0.03 to about 0.15 mol per liter of the electrolytesolution is dissolved. The amount of the ammonium halide that isdissolved is, for example, about 0.1 mol, 0.15 mol, 0.2 mol, 0.25 mol,0.3 mol, 0.35 mol, or 0.4 mol per liter of the electrolyte solution,where any value can form an upper end point or a lower end point, asappropriate. The amount of the alkali metal halide that is dissolved is,for example, about 0.03 mol, 0.04 mol, 0.05 mol, 0.06 mol, 0.07 mol,0.08 mol, 0.09 mol, 0.1 mol, 0.11 mol, 0.12 mol, 0.13 mol, 0.14 mol, or0.15 mol per liter of the electrolyte solution, where any value can forman upper end point or a lower end point, as appropriate. A dull-brightcoating is formed from the electrolyte solution including the additivesat block 1114.

Example 1

Iron (II) sulfate heptahydrate in the amount of about 30 g (about 0.11mol) per liter of electrolyte solution to be formed is dissolved inwater, which results in a light green solution. Although iron (II)sulfate heptahydrate was used in this example, one or more otherdivalent iron salts (e.g., iron (II) sulfate with other hydration state,iron (II) bromide, iron (II) chloride, iron (II) iodide, iron (II)nitrate, iron (II) acetate, iron (II) perchlorate, or other divalentiron salt) can be used instead of, or in addition to, chromium (III)chloride. Then, sodium citrate dihydrate in the amount of about 85 grams(about 0.29 mol) per liter of the electrolyte solution to be formed isdissolved in the light green solution, which results in a dark greensolution. Although sodium citrate dihydrate was used in this example,one or more other alkali metal citrates (e.g., sodium citrate with otherhydration state, other trisodium citrate, disodium hydrogen citrate,sodium dihydrogen citrate, potassium citrate, or other alkali metalcitrate) can be used instead of, or in addition to, sodium citrate.Then, sodium tungstate dihydrate in the amount of about 130 grams (about0.27 mol) per liter of the electrolyte solution to be formed isdissolved in the dark green solution, which forms a yellow-brownsolution. Although sodium tungstate dihydrate was used in this example,one or more other tungstate salts (e.g., sodium tungstate with otherhydration state, potassium tungstate, or other tungstate salt) can beused instead of, or in addition to, sodium tungstate dihydrate. Then,anhydrous citric acid in the amount of about 33 grams (about 0.26 mol)per liter of the electrolyte solution to be formed is dissolved in theyellow-brown solution, which forms a dark brown solution. Althoughanhydrous citric acid was used in this example, one or more other citricacid (e.g., citric acid monohydrate or other citric acid) can be usedinstead of, or in addition to, anhydrous citric acid.

Example 2

The resulting electrolyte solution of Example 1 was pH adjusted to about7.5. A mild steel substrate was plated by passing a direct current usinga platinized titanium mesh anode. The current density was maintained atabout 0.01 A/cm², the temperature was maintained at about 60° C., andthe pH was maintained at about 7.5. The pH adjustment may be done, forexample, by adding small amounts of sodium hydroxide, potassiumhydroxide or sulphuric acid. A bright hard coating was formed on themild steel substrate as shown in FIG. 12A. A scanning electronmicroscopy (SEM) image of the bright hard coating of FIG. 12A is shownin FIG. 13A. FIG. 14 shows an SEM image of a cross-section of amild-steel substrate 1402 with a bright hard coating 1404. Bright hardcoating 1404 is dense and compact.

Example 3

The resulting electrolyte solution of Example 1 was pH adjusted to about7.5. A mild steel substrate was plated by passing a direct current usinga platinized titanium mesh anode. The current density was maintained atabout 0.025 A/cm², the temperature was maintained at about 60° C., andthe pH was maintained at about 7.5. A bright hard coating was formed onthe mild steel substrate. An SEM image of the bright hard coating isshown in FIG. 13B.

Example 4

The resulting electrolyte solution of Example 1 was pH adjusted to about7.5. A mild steel substrate was plated by passing a direct current usinga platinized titanium mesh anode. The current density was maintained atabout 0.05 A/cm², the temperature was maintained at about 60° C., andthe pH was maintained at about 7.5. A dull coating was formed on themild steel substrate. An SEM image of the dull coating is shown in FIG.13C.

Example 5

The resulting electrolyte solution of Example 1 was pH adjusted to about7.5. A mild steel substrate was plated by passing a direct current usinga platinized titanium mesh anode. The current density was maintained atabout 0.075 A/cm², the temperature was maintained at about 60° C., andthe pH was maintained at about 7.5. A dull coating was formed on themild steel substrate as shown in FIG. 12B. An SEM image of the dullcoating of FIG. 12B is shown in FIG. 13D.

As illustrated by Examples 2-5, different types of iron-tungstencoatings can be formed based on the current density even when theelectrolyte solutions with the same content are used. Thus, the currentdensity to be applied can be selected to obtain the desired coatingcharacteristics (e.g., bright vs. dull).

Example 6

The resulting electrolyte solution of Example 1 was pH adjusted to about7.5. A brass substrate was plated by passing a direct current using aplatinized titanium mesh anode. The current density was maintained atabout 0.01 A/cm², the temperature was maintained at about 60° C., andthe pH was maintained at about 7.5. A bright hard coating was formed onthe brass substrate as shown in FIG. 15A. An SEM image of the brighthard coating of FIG. 15A is shown in FIG. 16A.

The appearance of the bright hard coating formed on the brass substrateas shown in FIG. 15A is similar to the bright hard coating formed on themild steel substrate as shown in FIG. 12A. Further, the microstructure(e.g., the surface topography) of the bright hard coating formed on thebrass substrate as shown in FIG. 15B is similar to the bright hardcoating formed on the mild steel substrate as shown in FIG. 12B.

Example 7

The resulting electrolyte solution of Example 1 was pH adjusted to about7.5. A brass substrate was plated by passing a direct current using aplatinized titanium mesh anode. The current density was maintained atabout 0.075 A/cm², the temperature was maintained at about 60° C., andthe pH was maintained at about 7.5. A dull coating was formed on thebrass substrate as shown in FIG. 15B. An SEM image of the dull coatingof FIG. 15B is shown in FIG. 16B.

The appearance of the dull coating formed on the brass substrate asshown in FIG. 15B is similar to the dull coating formed on the mildsteel substrate as shown in FIG. 12B. Further, the microstructure (e.g.,the surface topography) of the dull coating formed on the brasssubstrate as shown in FIG. 15B is similar to the dull coating formed onthe mild steel substrate as shown in FIG. 12D.

As illustrated by Examples 2-7, bright hard iron-tungsten coatings anddull iron-tungsten coatings can be formed on various substrates.Although mild steel substrates and brass substrates were used inExamples 2-7, the substrates are not limited to these substrates andvarious other substrates can be plated with an iron-tungsten coating.For example, the substrate can be other steel substrates, coppersubstrates, brass substrates, copper-coated substrates, nickel-coatedsubstrates or other metal or metal alloy substrates.

Example 8

The resulting electrolyte solution of Example 1 was pH adjusted to about7.5. A mild steel substrate was plated for about 2 hours by passing apulsed current using a platinized titanium mesh anode. The averagecurrent density was about 0.01 A/cm², the peak current density was about0.0115 A/cm², and the duty cycle was 87% (20/23) with a time on (t_(on))of 20 ms and a time off (t_(off)) of 3 ms. The temperature wasmaintained at about 60° C., and the pH was maintained at about 7.5. Abright hard coating was formed on the mild steel substrate as shown inFIG. 17. An SEM image of the bright hard coating of FIG. 17 is shown inFIG. 18. FIG. 19 shows an energy-dispersive X-ray spectroscopy spectrumof the bright hard coating of FIG. 17. Visual inspection of the brighthard coatings shown in FIG. 17 indicates smooth and bright hard coatingssimilar to those formed using direct current as shown in FIG. 12A. Also,the microstructure (e.g., the surface topography) of the bright hardcoating formed using pulsed current as shown in FIG. 18 was similar tothose formed using direct current as shown in FIG. 13A. However, thereis slight increase in the tungsten content (25.17 At %) of the coatingformed using pulsed current as compared to the coatings (22.7 At %)formed using direct current when electrolyte solutions with the samecontent is used. Further, pulsed current deposition at different currentdensities can be advantageously used for depositing multiple layers withvarying Fe:W ratio and/or varying hardness/modulus from the sameelectrolyte solution.

Example 9

The resulting electrolyte solution of Example 1 was used to plate a Hullcell panel. The current applied was about 1 Ampere, and the temperaturewas maintained at 55° C. The initial current was 0.9 A and the finalcurrent was also 0.9 A. The initial voltage was about 5.6 V and thefinal voltage was 5.3 V. The plating was performed for 5 min.

The 1 ampere Hull cell panel coated from the electrolyte solution ofExample 1 is shown in FIG. 20A. The current density decreases from leftto right. A dark/black dull coating was obtained at high currentdensities (the left), a dark dull coating was obtained at mid-currentdensities (the middle), and a bright coating was obtained at lowercurrent densities (the right).

Example 10

Iron (II) sulfate heptahydrate, sodium citrate dihydrate, sodiumtungstate dihydrate, and anhydrous citric acid were dissolved in anaqueous media as described in Example 1. Further, about 17 g (0.32 mol)of ammonium chloride and about 9 g of sodium bromide (about 0.087 mol)per liter of electrolyte solution to be formed was dissolved. Althoughammonium chloride was used in this example, one or more other ammoniumhalide can be used instead of, or in addition to, ammonium chloride.Also, although sodium bromide was used in this example, one or moreother alkali metal halide can be used instead of, or in addition to,sodium bromide.

Example 11

The resulting electrolyte solution of Example 10 was used to plate aHull cell panel. The current applied was about 1 Ampere, and thetemperature was maintained at 55° C. The plating was performed for 5min.

The 1 ampere Hull cell panel coated from the electrolyte solution ofExample 10 is shown in FIG. 20B. The current density decreases from leftto right. A dull-bright coating was obtained across all currentdensities.

Example 12

The resulting electrolyte solution of Example 10 was used to plate aHull cell panel. The current applied was about 5 Ampere, and thetemperature was maintained at 55° C. The plating was performed for 5min.

The 5 ampere Hull cell panel coated from the electrolyte solution ofExample 10 is shown in FIG. 20C. The current density decreases from leftto right. A dull-bright coating was obtained across all currentdensities with a wider coverage across the panel.

As illustrated by Examples 10-12, a substrate can be plated from anelectrolyte solution with additives such as ammonium chloride and sodiumbromide to form an iron-tungsten coating that is dull-bright across awider range of current densities and cover larger areas more uniformly.

When introducing elements of the present invention or exemplary aspectsor embodiment(s) thereof, the articles “a,” “an,” “the,” and “said” areintended to mean that there are one or more of the elements. The terms“comprising,” “including,” and “having” are intended to be inclusive andmean that there can be additional elements other than the listedelements. Although this invention has been described with respect tospecific embodiments, the details of these embodiments are not to beconstrued as limitations. Different aspects, embodiments and featuresare defined in detail herein. Each aspect, embodiment or feature sodefined can be combined with any other aspect(s), embodiment(s) orfeature(s) (preferred, advantageous or otherwise) unless clearlyindicated to the contrary. Embodiments described above illustrate but donot limit the invention. It should also be understood that numerousmodifications and variations are possible in accordance with theprinciples of the present disclosure. Accordingly, the scope of theinvention is defined only by the following claims.

What is claimed is:
 1. A method for plating a substrate using anelectrolyte solution, the method comprising: preparing the electrolytesolution consisting of an aqueous medium; a divalent iron salt; analkali metal citrate; a tungstate salt; a citric acid; and sodiumhydroxide, potassium hydroxide, sulfuric acid, or a combination thereofby: dissolving in the aqueous medium, the divalent iron salt in anamount ranging from about 0.05 to about 0.5 mol per liter of theelectrolyte solution and the alkali metal citrate in an amount rangingfrom about 0.05 to about 2 mol per liter of the electrolyte solution toform a first solution; dissolving in the first solution the tungstatesalt in an amount ranging from about 0.1 to about 1.5 mol per liter ofthe electrolyte solution to form a second solution; and dissolving inthe second solution the citric acid in an amount ranging from about 0.01to about 1 mol per liter of the electrolyte solution to form theelectrolyte solution; passing a current between a cathode and an anodethrough the electrolyte solution to deposit iron and tungsten on thesubstrate; and forming a coating layer comprising an alloy of iron andtungsten when the pH of the electrolyte solution is maintained at about3 to about
 12. 2. The method of claim 1, wherein: the step of dissolvingthe divalent iron salt and the alkali metal citrate comprises dissolvingiron (II) sulfate in an amount ranging from about 0.05 to about 0.2 molper liter of the electrolyte solution and sodium citrate or potassiumcitrate in an amount ranging from about 0.15 to about 0.5 mol per literof the electrolyte solution; the step of dissolving the tungstate saltcomprises dissolving sodium tungstate in an amount ranging from about0.15 to about 0.5 mol per liter of the electrolyte solution; and thestep of dissolving the citric acid comprises dissolving the citric acidin an amount ranging from about 0.02 to about 0.5 mol per liter of theelectrolyte solution.
 3. The method of claim 1, wherein the step ofpassing the current is performed using a carbonaceous anode, a graphiteanode, a platinum anode, or a platinized titanium anode to deposit ironand tungsten on the substrate comprising a steel substrate, a coppersubstrate, a brass substrate, a copper-coated substrate, a nickel-coatedsubstrate, or a combination thereof.
 4. The method of claim 1, whereinthe step of passing the current comprises applying a direct currenthaving a current density ranging from about 0.002 to about 0.04 A/cm2 toform a coating layer comprising an alloy of iron and tungsten having atungsten content ranging from about 18 to about 28 At %.
 5. The methodof claim 1, wherein the step of passing the current comprises applying adirect current having a current density ranging from about 0.05 to about0.1 A/cm2 to form a coating layer comprising an alloy of iron andtungsten having a tungsten content ranging from about 5 to about 18 At %or about 28 to about 40 At %.
 6. The method of claim 1, wherein the stepof dissolving the tungstate salt comprises dissolving the tungstate saltin an amount ranging from about 0.3 to about 0.45 mol per liter of theelectrolyte solution to form a coating layer comprising an alloy of ironand tungsten having a tungsten content ranging from about 18 to about 28At %.
 7. The method of claim 1, wherein the step of dissolving thetungstate salt comprises dissolving the tungstate salt in an amountranging from about 0.1 to about 0.3 mol per liter of the electrolytesolution to form a coating layer comprising an alloy of iron andtungsten having a tungsten content ranging from about 5 to about 18 At %or about 28 to about 40 At %.
 8. The method of claim 1, furthercomprising maintaining a temperature of about 50 to about 70° C. duringthe step of passing the current to form a coating layer comprising analloy of iron and tungsten having a tungsten content ranging from about18 to about 28 At %.
 9. The method of claim 1, further comprisingmaintaining a temperature of about 20 to about 50° C. during the step ofpassing the current to form a coating layer comprising an alloy of ironand tungsten having a tungsten content ranging from about 5 to about 18At % or about 28 to about 40 At %.
 10. The method of claim 1, whereinthe pH is maintained at about 7 to about 12 to form a coating layercomprising an alloy of iron and tungsten having a tungsten contentranging from about 18 to about 28 At %.
 11. The method of claim 1,wherein the pH is maintained at about 3 to about 7 to form a coatinglayer comprising an alloy of iron and tungsten having a tungsten contentranging from about 5 to about 18 At % or about 28 to about 40 At %. 12.The method of claim 1, wherein the step of dissolving the citric acidcomprises dissolving the citric acid in an amount ranging from about0.05 to about 0.25 mol per liter of the electrolyte solution to form acoating layer comprising an alloy of iron and tungsten having a tungstencontent ranging from about 18 to about 28 At %.
 13. The method of claim1, wherein the step of dissolving the citric acid comprises dissolvingthe citric acid in an amount ranging from about 0.25 to about 0.5 molper liter of the electrolyte solution to form a coating layer comprisingan alloy of iron and tungsten having a tungsten content ranging fromabout 5 to about 18 At % or about 28 to about 40 At %.
 14. The method ofclaim 1, wherein the step of passing the current comprises: applying afirst direct or pulsed current having a first current density during atleast one time interval to form a coating layer having a tungstencontent ranging from about 5 to about 18 At % or about 28 to about 40 At%; or applying a second direct or pulsed current having a second currentdensity during at least one second time interval to form a coating layerhaving a tungsten content ranging from about 18 to about 28 At %.
 15. Amethod for plating a substrate using an electrolyte solution, the methodcomprising: preparing the electrolyte solution consisting of an aqueousmedium; a divalent iron salt consisting of iron (II) sulfate, iron (II)halide, iron (II) nitrate, iron (II) acetate, or iron (II) perchlorate;an alkali metal citrate; a tungstate salt consisting of an alkali metaltungstate; a citric acid; and sodium hydroxide, potassium hydroxide,sulfuric acid, or a combination thereof by: dissolving in the aqueousmedium, the divalent iron salt in an amount ranging from about 0.05 toabout 0.5 mol per liter of the electrolyte solution and the alkali metalcitrate in an amount ranging from about 0.05 to about 2 mol per liter ofthe electrolyte solution to form a first solution; dissolving in thefirst solution the tungstate salt in an amount ranging from about 0.1 toabout 1.5 mol per liter of the electrolyte solution to form a secondsolution; and dissolving in the second solution the citric acid in anamount ranging from about 0.01 to about 1 mol per liter of theelectrolyte solution to form the electrolyte solution; passing a currentbetween a cathode and an anode through the electrolyte solution todeposit iron and tungsten on the substrate; and forming a coating layercomprising an alloy of iron and tungsten when the pH of the electrolytesolution is maintained at about 3 to about
 12. 16. The method of claim15, wherein the step of passing the current forms a plurality ofiron-tungsten alloy coating layers comprising at least one coating layercomprising an alloy of iron and tungsten having a tungsten contentranging from about 5 to about 18 At % or about 28 to about 40 At % andat least one coating layer comprising an alloy of iron and tungstenhaving a tungsten content ranging from about 18 to about 28 At %. 17.The method of claim 16, wherein a thickness of the at least one coatinglayer comprising an alloy of iron and tungsten having a tungsten contentranging from about 5 to about 18 At % is equal to a thickness of the atleast one coating layer comprising an alloy of iron and tungsten havinga tungsten content ranging from about 18 to about 28 At %.
 18. Themethod of claim 16, wherein a thickness of the at least one coatinglayer comprising an alloy of iron and tungsten having a tungsten contentranging from about 5 to about 18 At % is different from a thickness ofthe at least one coating layer comprising an alloy of iron and tungstenhaving a tungsten content ranging from about 18 to about 28 At %. 19.The method of claim 15, wherein the coating layer comprising an alloy ofiron and tungsten having a tungsten content ranging from about 18 toabout 28 At % is formed, and the pH of the electrolyte solution ismaintained at about 7.5.
 20. The method of claim 15, wherein the coatinglayer comprising an alloy of iron and tungsten having a tungsten contentranging from about 5 to about 18 At % or about 28 to about 40 At % isformed, and the pH of the electrolyte solution is maintained at about 3or about 5.